Image forming apparatus and devices such as a copying machine, a printer or a facsimile machine may use an electrophotographic system to heat and fuse a developer image that has been transferred from an image bearing body to a sheet of media, such as paper or a transparency resin sheet, and “fix” the image to a surface of the sheet. The transferring body may comprise nip rolls or a belt assembly. These devices preferably operate at high speeds to produce multiple copies rapidly. In doing so, a significant amount of heat energy is transferred to the sheet media as the fuser for the toner being transferred may operate in the range of about 130° C. to about 220° C. depending on the media transit speed and the nature of toner being transferred. The result is an accumulation of heat energy in the output bin of the apparatus as the media sheets are stacked.
With transparency media, which may be comprised of one or more layers of thermoplastic in sheet form, more heat is absorbed than plain paper. The higher thermal capacitance of transparencies may: (a) allow more energy to be “piled” into a stack of transparencies in the output bin; and/or (b) make an abnormally high temperature interface even between only two transparencies printed.
A problem in image quality may arise as multiple transparencies stack up in the printer output bin, as the heat from the fuser process accumulates and may cause the transferred toner to become “sticky” and adhere somewhat to the backside of the previous transparency which it rests against in the stack. Also, with the high thermal capacitance of transparencies even two sheets printed alone may be very hot when touching each other in the output bin. This may result in a residual image (offset) that appears on the backside of the earlier-copied sheet and/or voided areas in the toner image printed on the front side of the more recently copied sheet.
What is therefore needed is a process and a system that can adjust media variables such as process speed and fuser temperature to limit the rate of energy accumulated and allow more cooling time in a stack of media, e.g. transparencies, to below an energy/temperature threshold level and which will provide adequate fusing of the toner yet substantially prevent residual image transfer or “offsetting”.
What is also needed is a control algorithm which upon recognition of the type of media being processed initiates a process protocol to effectively control the accumulation of heat in the output bin or other collection location or will allow sufficient time or pause between as little as two media sheets, e.g. transparencies, such that a targeted interface temperature is not exceeded and bonding between the plastic film and toner does not occur in the stack.
It is thus an object of the present invention to provide a system and method of limiting the amount of media energy accumulated, or pausing sufficiently (e.g., increase in interpage gaps or even slow down of the rate of media transport) to allow cooling between the sheet-to-sheet interface of 2 or more media sheets in a stack output bin of a printer or like device.
It is a further object of the present invention to provide a control algorithm for controlling the heat accumulated in a stack of printed transparencies below a critical threshold level such that offsetting is substantially prevented.
It is further object of the present invention to provide a system and a method to reduce the rate of energy accumulated in a stack, or allow more time between even two printed sheets such as printed transparencies by, among other things, reducing the process speed of the media moving through the printing apparatus optionally in concert with reducing the fuser temperature when, e.g., transparencies are printed, to provide more cooling time yet still provide adequate fusing of the toner.
It is a still further object of the present invention to include an interval or pause time in the printing sequence as another technique of adjusting media throughput to prevent accumulation of energy in the output bin from exceeding a critical threshold value and to allow the energy accumulated in the output bin to dissipate.
These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show an embodiment in accordance with the present invention.